Chemguide: Support for CIE A level Chemistry

Learning outcomes 22.1(a) and 22.1(b)

This statement is about two sorts of chromatography.

Before you go on, you should find and read the statement in your copy of the syllabus.

Thin layer chromatography: TLC

This is just like the paper chromatography you may be familiar with, except that you use something other than paper.

Read the page about thin layer chromatography.

Make sure that you fully understand why some things move further than others during the chromatography.

It is important that you understand about Rf values because they are specifically mentioned in the syllabus, but it would be dangerous not to fully understand the rest of the page!

Gas-liquid chromatography

Read the whole page about gas-liquid chromatography so that you you understand the background to gas-liquid chromatography.

Make sure that you understand the importance of retention time, because that is specifically mentioned in the syllabus. You could safely ignore all the experimental details, but you should be aware of the reasons that substances will have different retention times.

Statement 22.1(b)

This asks you to work out percentage composition of a mixture from a gas-liquid chromatogram. You would be likely to get a simplified diagram like this, and you would need to measure it on the exam paper. The diagram shows the sort of results that you might get. Measure it as accurately as you can.

The numbers are in mm. (The diagram is only a sketch. Please don't tell me if you measure them and find they are something slightly different!)

What you need to do is to find the areas under the two peaks, add them together, and then work out the percentages. To do that, you need to remember a simple bit of geometry - the formula for the area of a triangle. That's 1/2 x base x height.

So in this case, the two areas are

1/2 x 22 x 15 = 165 mm2

1/2 x 15 x 30 = 225 mm2

That gives a total area of 390 mm2. From this, you can work out the percentage of the total area due to each component.

165/390 x 100 = 42.3%

225/390 x 100 = 57.7%

Don't expect more than 1 mark for doing this!

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© Jim Clark 2011 (last modified July 2014)